True, that remains to be seen.Don't mind me spoiling but you will have to generate same amount of terrain to get new symbols whether you get ultimate now and mystcraft update later or ultimte and mystcraft at the same time.
Still, a valid point might be, that incompatibility between old mystcraft worlds and new mystcraft worlds leading to crashes.
Today I have seen one of the new Mystcraft villager traders in one of Direwolf's SMP lets play. They can trade symbol pages for players to learn specific symbols. I guess server owners could also try to spawn such villagers by cheating them in, in some quantity, or by trying to breed them, if their map already has a substantial amount of world gen done already.